This invention relates to a phosphate system optical glass which is excellent in resistivity to devitrification, light transmissivity in ultraviolet and visible regions and homogeneity and has negative anomalous partial dispersion of a flint glass type.
If a design of an optical system such as a design of an object lens for a camera or a microscope, it is desired to remove secondary spectrum over a wide wavelength range. For this purpose, it is known to be effective to use a glass which has a smaller partial dispersion ratio (.theta.g.multidot.F) than a glass of normal dispersion having a linear proportional relation between Abbe's number (.nu.d) and partial dispersion ratio (.theta.g.multidot.F) and therefore has negative anomalous partial dispersion (-.DELTA..nu.d).
As a glass having negative anomalous partial dispersion, U.S. Pat. No. 2,684,304 discloses a P.sub.2 O.sub.5 -PbO system glass containing an extremely large amount of PbO ingredient and practically an extremely small amount of Al.sub.2 O.sub.3 ingredient. This glass, however, is so poor in resistivity to devitrification that it is difficult to obtain a sufficiently clear glass. Japanese Patent Publication No. 36-16726 discloses a P.sub.2 O.sub.5 -B.sub.2 O.sub.3 -PbO-Al.sub.2 O.sub.3 system glass. This glass, however, is not sufficient in resistivity to devitrification so that it is not suitable for mass production and, besides, this glass is poor in light transmissivity in the ultraviolet and visible regions. Japanese Preliminary Patent Publication No. 59-3044 discloses a B.sub.2 O.sub.3 -Sb.sub.2 O.sub.3 -ZnO-SiO.sub.2 -Al.sub.2 O.sub.3 -ZrO.sub.2 system glass. This glass, however, tends to be tinted yellow and besides it is difficult to homogenize this glss in melting it.
In phosphate system glasses, there is known a glass disclosed in Japanese Patent Publication No. 38-8459 which is of a P.sub.2 O.sub.5 -B.sub.2 O.sub.3 -(PbO/Sb.sub.2 O.sub.3 /Bi.sub.2 O.sub.3 etc.) system. It is difficult, however, to obtain anomalous partial dispersion of a desired value in this glass. Besides, this glass is inferior in light transmissivity and chemical durability.
It is therefore an object of the present invention to eliminate the above described disadvantages of the prior art glasses and provide an optical glass which has a value within a range of .nu.d=45 to 25 which is within a flint glass region and a negative anomalous partial dispersion within a range of .vertline.-.DELTA..nu.d.vertline..gtoreq.0.1 and which is excellent in resistivity to devitrification, light transmissivity in the ultraviolet and visible regions and homogeneity.